HIGH VOLTAGE Insulation Coordination Assistant Professor Suna BOLAT KRÖGER

Eastern Mediterranean University

Department of Electric & Electronic Engineering

Insulation coordination

The term Insulation Co-ordination was originally introduced to arrange the insulation levels of the several components in the transmission system in such a manner that an insulation failure, would be confined to the place on the system where it would

• result in the least damage,

• be the least expensive to repair,

• and cause the least disturbance to the continuity of the supply.

The present usage of the term

• Insulation co-ordination now comprises the selection of the electric strength of equipment in relation to the voltages which can appear on the system for which the equipment is intended.

• The overall aim is to reduce to an economically and operationally acceptable level the cost and disturbance caused by insulation failure and resulting system outages.

• To keep interruptions to a minimum, the insulation of the various parts of the system must be so graded that flashovers only occur at intended points.

• With increasing system voltage, the need to reduce the amount of insulation in the system, by proper co-ordination of the insulating levels become more critical.

Terminology

• Nominal System Voltage: It is the r.m.s. phase-to-phase voltage by which a system is designated

• Maximum System Voltage: It is the maximum rise of the r.m.s. phase-to-phase system voltage

• Insulation Level: For equipment rated at less than 300 kV, it is a statement of the Lightning impulse withstand voltage and the short duration power frequency withstand voltage.

For equipment rated at greater than 300 kV, it is a statement of the Switching impulse withstand voltage and the power frequency withstand voltage. • Protective Level of Protective Device: These are the highest peak voltage

value which should not be exceeded at the terminals of a protective device when switching impulses and lightning impulses of standard shape and rate values are applied under specific conditions.

Method of insulation coordination

• In order to avoid insulation failure, the insulation level of different types of equipment connected to the system has to be higher than the magnitude of transient overvoltages that appear on the system.

• The magnitude of transient over-voltages are usually limited to a protective level by protective devices.

• Thus the insulation level has to be above the protective level by a safe margin.

• Normally the impulse insulation level is established at a value 15-20% above the protective level.

Insulation level

Protection level

• Correlation of the insulation strength of electrical

equipment with the characteristics of protective

devices such that the insulation is protected from

excessive overvoltages

Overvoltage protection

• Rod and horn gaps: Flash over with overvoltage -

Fault current flows - Outage- Poor protection for short ,

high impulses

Overvoltage protection

• Silicon carbide gapped lightning arresters:

Nonlinear resistor in series with gap - gap flashes over -

current and voltage limited by resistor - power arc goes

out at current zero - magnetic blow out coils.

Conclusion

• Insulation Coordination encompasses all aspects of the power system and attempts to ensure uninterrupted supply of power under the worst overvoltage and environmental conditions.

• Surge arresters play an important role in reaching this goal.

Overvoltages

• Switching overvoltages

U [kV]

t [s]

Overvoltages

• Lightning overvoltages

U [kV]

t [s]

Lightning

• An electrical discharge occurring in a big electrode seperation with high current, high voltage

Thunder clouds

cumulonimbus

Diameter: about 10 km Height: about 14-15 km

Average electric field on earth

• Good weather: 100 V/m = 0,1 kV/m = 0,001 kV/cm

• Thunderstorm: 15-20 kV/m

• Field under the thunder cloud: 1000 -10000 V/m = 1 kV/m -10 kV/m

Charge formation

Lightning types

• Cloud to ground

• Cloud to cloud

• Inside cloud

• Cloud to sky

Cloud to ground lightnings

• Positive downward

• Negative downward

• Positive upward

• Negative upward

Properties of lightning

• Voltage: 1 MV – 1000 MV

• Current: 3 kA – 400 kA (in practice ave: 18 kA)

• Power: P = U * I : MW (BIG!)

• Energy: W = P * t : J or Ws (SMALL)

• Temperature: 25000 – 30000 ° K (hotter than sun!)

• Thickness: 1 – 10 cm

• Charge: 0.1 – 10 C

Effects of lightning

• Thermal effect (Joule los, heat, fire, melting)

• Thermodynamic effect, electrodynamic effect (deformation)

• Electromagnetic effect (induction)

• Acoustic effect (thunder)

• Visual effect (flash)

• Electrochemical effect (Ozone, NO)

• Effects on living (dangerous burns, muscular contraction, fibrilation, step voltage)

Impulse voltage

Power = U x I

= 100 kV x 20 kA

= 2000 MW (MegaWatt)

Energy = P x t

=2000 MW x 10 µs

=200 Ws = 200 J

Impulse voltage

Impulse current

Lightning rod (French: Paratonneire, Old synonym: Siperisaika)

It is a set-up for protection against effects of lightning. Research done by Buffon, Romas and Franklin is resulted in the invention of "Franklin rod" still used today.

The system consists of a metal rod, a wire connecting it to the ground conductor and the ground conductor that distributes the current. Louis Mertens developed a method in 19th century, called a Faraday cage. The building to be protected is surrounded by conductors.

Later, using radioactive elements at the sharp edges, modern protection devices are developed. Today, radioactive lightning rods are abandoned and replaced with non-radioactive rods in order to avoid a possible radioactivity.

Protection wire, Protection line, protective earth conductor

protection angle protection zone

Michael Faraday (1791-1867) Faraday cage